As world reserves of light, sweet crudes diminish and worldwide consumption of oil increases, refiners seek methods for extracting useful products such as gasoline and fuel oils from heavier crude resources. While not as desirable and easy to process, extensive reserves in the form of “heavy crudes” exist in a number of countries, including Western Canada, Venezuela, Russia, the United States, and elsewhere.
Such heavy oils (even some not so heavy oils) are often difficult to refine because of their viscosity and propensity for being unstable and precipitating solids, such as asphaltenes, coke, coke precursors, etc. upon storage and processing, most notable asphaltenes. Asphaltenes are most commonly defined as that portion of petroleum, which is soluble in xylene and toluene, but insoluble in heptane or pentane. Asphaltenes exist in crude oil as both soluble species and in the form of colloidal dispersions stabilized by other components in the crude oil. Asphaltenes have higher molecular weights and are the more polar fractions of crude oil, and can precipitate upon pressure, temperature, and compositional changes in crude oil resulting from blending or other mechanical or physicochemical processing. Asphaltene precipitation and deposition can cause problems in subterranean reservoirs, upstream production facilities, mid-stream transportation facilities, refineries, and fuel blending operations. In petroleum production facilities, asphaltene precipitation and deposition can occur in near wellbore reservoir regions, wells, flowlines, separators, and other equipment.
When asphaltenes precipitate from crude oil, they can foul equipment and reduce the quality of the products being refined. Other issues associated with heavy crude oil include: high solids; increased amounts of entrained water; and high sulfur content; high total acid number (TAN) and high metals. Asphaltene deposition is a well-known problem affecting all aspects of petroleum production and processing. Crude oils containing high or low levels of asphaltenes can be destabilized while processing causing fouling, formation of sludge, corrosion and all the equipment fixing, cleaning, and cost aggravations associated with these effects.
Additional operational problems observed with heavy crude oil: difficulty in blending crude streams, increased unit upsets, increased pollution, loss of through-put, difficulty with desalting, increased load on wastewater plants, increase in air emissions, and flexibility in plant operations is reduced. All of this leads to an overall increase in operating costs.
Asphaltenes may be present and stable in a crude oil under equilibrium reservoir conditions, but may aggregate or deposit as temperatures, pressures, and overall fluid compositions change as the crude oil is removed from the reservoir during production and/or being further refined. Asphaltenes are typically dark brown to black-colored amorphous solids with complex structures and relatively high molecular weights.
Asphaltene stability can even be disturbed by mixing hydrocarbon-based fluids i.e. such as mixing two types of crude oils together, two types of shale oils together, condensates, and others, of different origins at certain ratios as the chemistry of the hydrocarbon-based fluids from different sources may be incompatible and induce destabilization of the asphaltenes therein. In non-limiting examples, such as during refining or fuel blending, two or more hydrocarbon-based fluids may be mixed together. Sometimes, changes in physical conditions are sufficient to induce destabilization, or even the mixture of different hydrocarbon-based fluids that have incompatible chemistries. Said differently, even if neither oil-based fluid, alone, has destabilized foulants or the hydrocarbon-based fluid would not act as a destabilizing additive by itself, the mixing or the mixture of two or more hydrocarbon-based fluids may further destabilize the foulants present in either hydrocarbon-based fluid.
There are several shortcomings when measuring asphaltene stability to improve foulant stability. Thus, it would be desirable to develop better methods of analyzing the stability of the asphaltenes within crude oils.